Billerica is a town in Middlesex County, Massachusetts, United States. The population was 40,243 according to the 2010 census. It is the only town named Billerica in the United States and borrows its name from the town of Billericay in Essex, England.

Billerica is located 20 miles (32 km) north-northwest of Boston along the Northwest Expressway (Massachusetts) portion of U.S. Route 3, positioning it as the border between the Boston Metro region to the south and the Greater Lowell region to the north. The town is also situated less than 3 miles from the Massachusetts Route 128/Interstate 95 High-Technology belt to the south and less than 2 miles from the Interstate 495 (Massachusetts) outer belt highway to the north. Billerica has several small neighborhoods that form villages (or sections) of town. Those villages are East Billerica, North Billerica, Nutting Lake, Pinehurst, Rio Vista, River Pines, Riverdale, Riverside, and South Billerica. The Shawsheen River and Concord River are the two major waterways within the town. Nuttings Lake offers a public beach and other recreational water activities.

As of the census of 2000, there were 38,981 people, 12,919 households, and 10,244 families residing in the town. The population density was 1,505.9 people per square mile (581.3/km≤). There were 13,071 housing units at an average density of 504.9 per square mile (194.9/km≤).